On this web page we present a limit on the mass splitting for the
hypothesis that the measured X(3872) signal is produced by two different states
with slightly different mass and we show an improved measurement of its mass.
The results were blessed in the B Physics Meeting on July 24, 2008.

We select X(3872) and ψ(2S) candidates in the decay mode J/ψππ using an
artificial neural network and a cut on the number of candidates per event.

To test the hypothesis whether the observed X(3872) signal stems from two different
states, we fit the X(3872) mass signal with a Breit-Wigner function convoluted
with a resolution function determined from simulation.
Both functions contain a width scale factor that is a free parameter in the fit
and therefore sensitive to the shape of the mass signal.
The measured width scale factor is compared to the values seen in pseudo experiments
which assume two states with given mass difference and ratio of events.
The resolution in the simulated events is corrected for the difference between
data and simulation measured for the ψ(2S).
The result of this hypotheses test shows that data is very well consistent with
only one state.
Under the assumption of two states with equal amount of observed events we set a limit of

Δ m < 3.2 (3.6) MeV/c2 at 90% (95%) C.L.

The limit for other ratios of events in the two peaks is shown in the following figure:

Since our signal is consistent with one peak we proceed and measure
the X(3872) mass in an unbinned maximum likelihood fit.
The systematic uncertainties are determined from the difference between
the measured ψ(2S) mass and its world average value, the potential variation
of the ψ(2S) mass as a function of kinematic variables and the difference
in Q value between X(3872) and ψ(2S).
Systematic effects due to the fit model are negligible.
The measured X(3872) mass is:

m(X(3872)) = 3871.61±0.16 (stat)±0.19 (syst) MeV/c2.

This is the most precise measurement to date.
The value is below, but within uncertainties of the D*D threshold.
The explanation of the X(3872) as a bound D*D system is therefore still an option.

Figure 4 (eps, gif): Optimization of cut on the number of candidates per event based on significance defined by number of signal events devided by square root of signal plus background events in the signal region in data

Figure 5 (eps, gif): Effect of the cut on the number of candidates on the mass spectrum

Figure 6 (eps, gif): Distribution of width scale factor in simulated events with Δm = 0 compared to the value seen in data